Rolled aluminum alloy in the form of sheet (unless otherwise indicated, sheet as referred to herein includes "foil") is a favored material for making lithographic plate ("lithoplate"), because of its cost effectiveness. But the lithoplate must be properly grained. By "lithoplate", we refer to the aluminum support material before it is coated with a photosensitive "resist". "Cost effectiveness" refers to the number of prints of acceptable quality which can be made with a single resist-coated lithoplate before it must be replaced. "Graining" refers to the roughening of a surface of the aluminum sheet. Graining the aluminum sheet is the first step towards providing photoresist-coated sheet with the requisite hydrophobic and hydrophilic characteristics which generate image and non-image areas. Though an aluminum alloy is used, commercial lithoplate of aluminum alloy is referred to as "aluminum" sheet or foil, for brevity, partially because nearly pure aluminum, such as 1050 alloy (99.5% pure) is a preferred material for electrochemically etched lithoplate, and partially because pure aluminum is known to be an impractical material for lithoplate.
To provide the hydrophobic and hydrophilic characteristics, a grained aluminum sheet is uniformly coated with a photosensitive "resist" composition which is exposed to actinic radiation beamed onto the resist through an overlay which corresponds to the image to be printed. Areas which are comparatively more soluble following irradiation must be capable of being easily removed from the support, by a developing operation, to generate the hydrophilic non-image areas without leaving a residue. The support which has been laid bare must be strongly hydrophilic during the lithographic printing operation, and be able to exert an adequately repelling effect with respect to the greasy printing ink.
The cost of producing lithoplate includes the cost of producing foil of an appropriately affordable alloy, the foil having a highly uniform microstructure, such as that obtained with electrochemical etching. The conventional wisdom has been: the more uniform the microstructure of controllably grained foil, the better suited the grained foil for use as lithoplate.
In addition to 1050 alloy, other widely used alloys are 3003, 1100 and 5XXX, the latter being specifically produced for the production of lithoplate, as disclosed in U.S. Pat. No. 4,902,353 to Rooy et al (class 148/subclass 2) the disclosure of which is incorporated by reference thereto as if fully set forth herein. Though the cost of such alloys themselves is not high relative to the value of the printed material generated with photoresist-coated lithoplate made therefrom, lithoplate is nevertheless deemed costly, and the ongoing challenge is to produce more cost-effective lithoplate.
The cost of lithoplate is ascribed in large part to the cost of graining aluminum sheet so that it is free from imperfections and will provide adequate resolution of the print to be made, as well as many hundreds, if not thousands of prints, before one must change the lithoplate in a printing press. Such imperfection-free graining, at present, is preferably accomplished by choice of an alloy which is particularly well adapted to chemical etching which is closely controlled by a bath composition, and the narrowly defined process conditions of its use. Together these result in the highly uniform graining, found by dint of experimentation under actual printing conditions, to be cost effective. Not only is the optimum aluminum alloy expensive, but so also is the necessary close control for chemical etching, and formulating and maintaining a chemical bath. Disposing of exhausted bath compositions further adds to the expense.
Such considerations militate towards finding a non-chemical solution to the problem of graining an aluminum sheet or foil for lithoplate. But non-chemical graining, that is, mechanical graining, is generally accepted as being too non-uniform, not only because it is relatively coarse compared to electrochemical etching, but also because it is difficult to control. The ongoing search is for a solution to the problem of providing controllably grained lithoplate without using an electrochemical process.
We have surprisingly found such a solution, except that it produces a highly non-uniform microstructure compared with that produced by electrochemical graining. It is believed that one skilled in the art of graining foil for use as lithoplate, upon viewing a photomicrograph of the non-uniform microstructure we produce on foil, simply would consider its use for lithoplate to be surprising.
In a resistance welding method disclosed in U.S. Pat. No. 4,633,054 (class 219/subclass 118), the disclosure of which is incorporated by reference thereto as if fully set forth herein, we prepared the surface of aluminum sheet for resistance welding. Such preparation involved several procedures, each of which was primarily directed to removing the surface oxide on a workpiece to be welded.
One of these procedures involved "arc cleaning" the surface of the workpiece with an electric arc, the intense heat of which contorted the planar configuration of the sheet. This arc cleaning, also referred to as "cathodic cleaning", resulted in roughening the contorted surface of the sheet in such a manner that the roughened surface was coarse and non-uniform, characterized by a high density of peaks rather than long ridges. Such arc cleaning is effected under conditions of electric current and traversal rate (the rate at which the arc traverses the sheet), which to a large extent overlap the conditions used for arc graining; except that, not all arc-cleaned sheet is suitable for being converted to lithoplate, but all uncoated arc-grained sheet is suitable for resistance welding.
All arc-grained aluminum sheet has a characteristic surface morphology, referred to as an "arc-grained morphology", characterized by a profusion of craters peripherally surrounded by delicate petal-like protrusions or projections. The craters range from about 1.mu. (micron)to about 10.mu. in diameter, typically in the range from about 2.mu. to about 5.mu. in diam. A substantial proportion, from about 30% to about 80%, or more of the protrusions, terminate in peaks or crenelations which, together with the craters, imbue such a surface, when coated with a phosphate-free protective coating, with a unique capillary action, namely the ability to have a capillary uptake of both water and printing ink.
These uncoated peaks were mechanically unstable and easily compressed when contacted with the electrode used to make the resistance weld (see '054 patent, col. 3, lines 45-50). For the specific purpose of resistance welding, the effectiveness of such arc cleaning of the surface of aluminum stock to be resistance welded was predicated upon the peaks being so high as to be easily compressible by mechanical pressure exerted by the welding electrode. The peculiarly delicate nature of the uncoated arc-cleaned surface improved the electrical contact between the electrode and the sheet, and resulted in the lowest interface resistance of the treatments evaluated, allowing effective resistance welding. Neither the coarseness nor the random undulations of the contorted surface were of much, if any, import as long as the interface resistance was sufficiently low. The object was to resistance weld one sheet to another adequately resistance-free aluminum surface. Sheets to be resistance welded are clamped to one another so that the random undulations in either surface are flattened out. Undulations in foil make it unusable as lithoplate.
Though such an arc-cleaned surface was just right for a workpiece to be resistance welded, an unstable delicate surface, with high peaks and correspondingly deep valleys, was microstructured so differently from the rugged, highly uniform, imperfection-free, fine-grained surface conceptualized as being the ideal lithoplate surface, it was to be expected that the arc-cleaned surface was deemed a most unlikely candidate for consideration in a lithoplate application. For one thing, even after being coated with a durable phosphate-free coating, a delicate surface will be quickly destroyed in normal use on a lithographic printing press. High mechanical stability is a well-established prerequisite for the supporting surface of lithoplate. For another, unless the configuration of anodized peaks and valleys, as well as the density or number of peaks, were both fortuitously matched to the required capillary uptake of conventional printing inks used in a printing press in the image areas, and of water in the non-image areas, there was no reason seriously to consider using an arc-cleaned aluminum surface for lithoplate, or modifying it to render it usable as lithoplate.
Nevertheless we did consider using an arc-cleaned aluminum sheet, and discovered we could modify the coarsely-grained surface by coating it, successfully enough to produce cost-effective lithoplate of remarkably high quality, and obtain a "run life" which exceeded our most optimistic expectations.